Non-aqueous electrolytic secondary batteries such as lithium ion secondary batteries are widely used as the main power supplies of portable electronic devices or power supplies for driving hybrid electric vehicles or the like since they are small and lightweight and have high energy density. In a lithium ion secondary battery, a separator is disposed between positive and negative electrodes. The separator prevents shorting due to contact between active materials of both polarities and holds an electrolytic solution in the pores thereof so as to form a pathway for ion conduction.
The burden on battery members has increased in step with increases in the energy density of lithium ion secondary batteries in recent years, and separators are also required to have a higher degree of safety performance. Therefore, a separator for a lithium ion secondary battery must not only have excellent mechanical strength, but must also exhibit excellent thermal shrinkage characteristics at high temperatures by demonstrating excellent results in high-temperature cycle tests, oven tests, and the like, which are tests for evaluating battery safety prescribed by U.S. Standard UL-1642 (Underwriters Laboratories) or the International Electrotechnical Commission Standard IEC-61960 (International Electrotechnical Commission), and the like when used in the form of a battery.
Polyolefin microporous membranes have long been used as separators for lithium ion secondary batteries. Among polyolefin microporous membranes, microporous membranes comprising polyethylene resins, in particular, are known to have an excellent shutdown function, whereby the micropores of the porous membrane are blocked so as to block the flow of current when the temperature of the battery increases.
However, the battery temperature may increase further after the shutdown function has been activated, and in such cases, the melting (so-called “meltdown”) of the separator progresses and causes shorting inside the battery. This generates a large amount of heat, which leads to the risk of smoke, fire, and explosion. Therefore, in addition to a shutdown function, there is a demand for a separator to have excellent heat resistance so that there is no risk of shorting even when a temperature higher than the temperature at which the shutdown function is activated is reached, and so that the risk of shorting is suppressed even when held for a certain amount of time at a temperature higher than the shutdown temperature.
Accordingly, in order to enhance the heat resistance of a separator, a polyolefin microporous membrane containing polypropylene, which has a higher melting point than polyethylene, has been proposed (for example, see Patent Document 1). However, although a microporous membrane containing a polypropylene resin has a high meltdown temperature, there has been a problem in that the shutdown temperature is also high.
In addition, in order to achieve both shutdown characteristics and meltdown characteristics, it has also been proposed to blend a polyethylene and a polypropylene or to laminate a microporous membrane comprising a polyethylene resin and a microporous membrane comprising a polypropylene resin.
For example, in Patent Document 2, in a polyolefin multilayer microporous membrane comprising at least three layers, the surface layers on both sides are layers comprising only a polyethylene resin, and an inner layer, which contains a polyethylene resin and a polypropylene having a heat of fusion (ΔHm) of at least 90 J/g when measured by differential scanning calorimetry, wherein the compounding ratio thereof is adjusted appropriately, is interposed between both polyethylene resin layers. In this case, it is disclosed that a polyolefin multilayer microporous membrane which exhibits a low shutdown temperature, high shutdown rate, and high meltdown temperature and has excellent film forming properties is obtained.
In addition, in Patent Document 3, a first microporous layer containing a first polyethylene resin in which the proportion of ultrahigh molecular weight polyethylene having a weight average molecular weight of at least 1×106 is at least 8 mass %, and a second microporous layer containing a second polyethylene resin in which the proportion of ultrahigh molecular weight polyethylene is at most 7 mass % and having a structure in which a pore distribution curve determined by a mercury penetration method has at least two peaks, are provided, wherein when the total thickness of the first and second microporous layers is defined as 100%, the thickness of the first microporous layer is from 15 to 60%. In this case, it is disclosed that a polyolefin multilayer microporous membrane having an excellent balance of permeability, mechanical strength, meltdown characteristics, electrolyte absorbency, and electrolyte retention is obtained.
Under such circumstances, there is a demand for the development of a polyolefin multilayer microporous membrane which solves the problems of conventional polyolefin multilayer microporous membranes and pursues the further enhancement of mechanical strength and heat resistance in step with the increasing performance of lithium ion secondary batteries, and a battery separator using the same.